Re: Question on dc-coupled pair using separate tubes...

 :

6/3/2000 4:37 AM

Steve Ahola

Re: Question on dc-coupled pair using separate tubes... Dave:     Ken was describing the cathodyne PI where the signals are taken from the cathode and plate of the tube so you would want 1/3 of B+ across each resistor.         OK, so with a rail voltage of, say, 300vdc you would want 200vdc on the plate (or 100vdc across the plate resistor)- right? I'm wiring up a cathodyne PI right now so that information is very helpful!     --Thanks!     Steve Ahola

 

6/5/2000 11:33 AM

Dave H.

With a 300V supply and 200V on the plate the plate to cathode voltage will only be 100V. I think it would work better with more volts across the tube. The one I have is the usual Fender circuit (56k plate and cathode resistors, 1k5 bias resistor) and it has a 320V supply to the PI, 255V on the plate and 65V on the cathode. It drives a couple of 5881s cathode biased at 27V OK but I’m not sure if there would be enough output to drive them fixed bias at 40V.     Dave

 

5/28/2000 9:47 PM
Peter S	Steve,   You can find the cathode-filament max voltage in the tube data sheet for a given tube.....for a typical 12ax7 it is about 180volts.   PS

 

5/27/2000 10:45 PM

Frank Clarke

Thanks Steve,   Your schematic is correct, I'll change mine so as not to scare people.   The 820 ohm version is more normal, I didn't want to go changing too much in one go.   I'll rig up a large test switch for a CF/non-CF, just to check that I actually like the difference. My ear has a bad memory.   I could use V2a as a extra input, something like the AX84 November (http://www.techaccessinc.com/ax84/november/nov_r8.pdf). That would keep the original sound (which is fine) but I could jumper the inputs for more gain. But then I'd be tempted to use V2b as the CF, and end up with a Fender Plexi.     I've just noticed that somebody on the AX84 Homebrewing Forum wants to add switches to a Fender Pro Junior to convert it into a November. They would be a bit short on preamp tubes with the Pro Junior, but you could permanently convert the Blues Junior to a plexi-like preamp.     If I do come up with something which preserves the original sound, but adds an optional Marshall-like vibe I'll keep y'all posted. No B+ on mini-toggles I promise!

 

<<First Page

<Prev

Page 3 of 3